Polycrystalline graphite with controlled electrical conductivity

ABSTRACT

An electrical resistance element which comprises polycrystalline graphite containing 0.08 weight percent of boron and which has an electrical resistivity that is constant within a 5 percent band over the temperature range 0*-2500* C.

United States Patent Wagner et al.

[54] POLYCRYSTALLINE GRAPHITE WITH CONTROLLED ELECTRICAL CONDUCTIVITY Inventors: Paul Wagner; James M. Dickinson; Morton C. Smith, all of Los Alamos, N. Mex.

' Assignee: The United States of America as represented by the United States Atomic Energy Commission Filed: Nov. 14, 1969 Appl. No.: 876,931

US. Cl ..252/503 Int. Cl. ..H0lb 1/04 Field oi Search ..252/502, 503, 507

[ Feb. 22, 1972 [56] References Cited UNiTED STATES PATENTS 2,949,430 8/ 1960 Jorgensen ..252/502 Primary Examiner-Douglas J. Drummond Attorney-Roland A. Anderson [57] ABSTRACT An electrical resistance element which comprises polycrystalline graphite containing 0.08 weight percent of boron and which has an electrical resistivity that is constant within a 5 percent band over the temperature range 0-2500 C.

1 Claims, 1 Drawing Figure ELECTRlCAL RESISTIVITY (#9. cm

w/o 8/6 0.041 4.54 1 io o.oe| 8.98

I I l o 500 I000 I500 2000 2500 TEMPERATURE, C

POLYCRYSTALLINE GRAPHITE WITH CONTROLLED ELECTRICAL CONDUCTIVITY The invention described herein was made in the course of, or under, a contract with the U. S. Atomic Energy Commission.

This invention relates to electrical resistance elements and more particularly to boron-containing polycrystalline graphite electrical resistance elements.

Electrical resistors composed of graphite are well known in the art. However, such resistors when used for heating purposes require special devices such as thermostats and the like to be used in conjunction with them for the purpose of controlling the temperature or keeping it constant. The reason for this is that the electrical resistivity of graphite varies according to the temperature. Thus, pure graphite has a negative temperature coefficient of resistivity between and 800 C., that is, as the graphite is heated in this temperature range the electrical resistivity steadily declines. Above about 800 C. there exists an ill-defined minimum, and the value of the electrical resistivity then increases with temperature at least to 2,500 C. Nonetheless, because of their relatively high resistivity and ease of fabrication, graphites continue to have utility as heating elements.

it is apparent that a resistance elementcomposedof a graphitic material exhibiting a constant voltage-current relationship over a wide temperature range is desirable in that it eliminates the necessity for the use of thermostats or other special control elements in a furnace or other heating system. Such material may also find widespread application in ballast resistances, electrodes, etc.

It is therefore an object of this invention to provide a material for use in electrical resistance elements that has an electrical resistivity sensibly constant over the temperature range 0-2,500 C. A further object is to provide electrical resistance elements having a relatively high resistivity that can be economically and easily fabricated.

The addition of boron to graphite has long been known to both enhance the graphitization process and affect the electrical properties of the graphite. A common statement in the literature is that addition of small concentrations of boron decreases the electrical resistivity. This observation is true, however, only at about room temperature, i.e., 23 C.

The FIGURE shows the effect of temperature on the electrical resistivity of pure and boron-doped polycrystalline graphites. lt can readily be seen that the electrical resistivity of graphite containing 0.08 weight percent boron is nearly constant between 0 and 2,500 C. The actual deviation from a constant resistivity over this temperature range has been found to be no more than fi percent.

In the preferred embodiment of this invention, the pure graphite and the boron-doped graphites used to obtain the measurements given in the Figure are prepared in the following fashion. The mix contains a filler of parts graphite flour, 15 parts Therrnax (carbon black) and 27 parts per hundred of Varcum resin (a partially polymerized furfural alcohol binder), maleic anhydride catalyst, and boron powder in amounts necessary to make up the desired concentrations. The dry ingredients are twin-shell blended, the catalyzed binder added, and the blending continued. The mix is then chopped five times in a hamburger grinder with one-eighthinch holes in the chopping plate. Next the mix is extruded through a one-half-inch-diameter die at about 42 C. The resulting rods are fed through the food chopper twice, again extruded, chopped twice more and finally vacuum extruded for the third time at 50 C. The rods are then cured by heating to 200 in air, baked in vacuum to a maximum temperature of 900, and graphitized at about 2,700 C. in a helium atmosphere. The method for preparing a polycrystalline graphite is similar to that reported in 7 Carbon 273 1968).

The boron dispersion in graphite prepared by this procedure is excellent. Electrical resistivities measured at linch intervals on one-half-inch-diameter extruded rods thus prepared had a standard deviation of less than 1 percent. The

electrical resistivity of a pure gra hite with uniform crystallographic structure and density wt 1 generally show a standard deviation of about 3 percent.

For boron concentrations of 0.16 weight percent and less, spectrochemical and X-ray diffraction analyses show no loss of boron either during the'graphitization process or after 5 hours at 2,500 C. It does appear that at some boron concentration between 0.l6 and 0.39 weight percent changes occur on heat treatment which indicate either migration or loss of boron.

A polycrystalline graphite containing about 0.08 weight percent boron that has an electrical resistivity constant within a 5 percent band over the temperature range 02,500 C. has thus been disclosed. There appear to be no temperature restrictions within this temperature range on the retention of the boron in the graphite. The fabrication and machinability of this material are essentially identical to that of pure graphite. There are also no apparent restrictions on the form which can be taken by a finished electrical resistance element composed of this material.

What we claim is:

l. A material having an electrical resistivity that remains sensibly constant from 0 to 2,500 C., said material comprising a polycrystalline graphite in which about 0.08 weight percent concentration of boron has been uniformly dispersed. 

